The size of a Solar Power System is determined in both area and performance. Performance is usually expressed in Wattage, but can also be a factor of Wattage per giving area, or more importantly, Wattage per Dollar Invested.
Ideally, you will want a System that is big enough to supply all the electrical load for your household or business, perhaps with some extra capacity to sell back to the utility company. You can determine how much power you need by studying your old utility bills.
If you cannot afford a Solar Power System which will completely meet your power needs, we still recommend that you purchase as large a system as you can afford.
You will enjoy the savings provided by the system you have, and you can expand the system later.